NL2011290C2

NL2011290C2 - A laser micromachining system for writing a pattern onto a substrate using laser micromachining and method.

Info

Publication number: NL2011290C2
Application number: NL2011290A
Authority: NL
Inventors: Jacobus Lambertus Merksteijn
Original assignee: Climate Invest B V
Priority date: 2013-08-12
Filing date: 2013-08-12
Publication date: 2015-02-16
Also published as: WO2015022333A2; WO2015022333A3

Abstract

The present invention is related to a system and method for writing a pattern onto a substrate using laser micromachining. The system comprises a seed laser system (11) for outputting a plurality of laser beams (a, b, c), each laser beam comprising a plurality of subsequent light pulses; a laser crystal (12) for amplifying light pulses; a pumping unit (13) for providing pumping power to the laser crystal; a first selector unit (16) configured for individually switching the laser beams outputted by the seed laser system between an active and inactive state, wherein, the first selector unit is configured to select, for each laser beam in the active state, a light pulse among the plurality of light pulses, to be passed towards the laser crystal for amplification; a light directing unit (14) for directing incident light pulses towards the substrate; a second selector unit (17) configured for selecting, for each laser beam outputted by the laser crystal, an amplified light pulse among the plurality of amplified light pulses, to be directed to the light directing unit for writing said pattern; a control unit (18) for controlling the pumping unit, the light directing unit and the first and second selector units. The method comprises the steps of: outputting a plurality of laser beams, each laser beam comprising a plurality of subsequent light pulses; providing a laser crystal for amplifying light pulses; providing pumping power to the laser crystal; individually switching the laser beams outputted by the seed laser system between an active and inactive state, wherein, for each laser beam in the active state, a light pulse among the plurality of light pulses is selected using a first selector unit to be passed towards the laser crystal for amplification; using a second selector unit for selecting, for each laser beam outputted by the laser crystal, an amplified light pulse among the plurality of amplified light pulses; directing the selected amplified light pulses towards the substrate for writing said pattern. According to the invention, individually controllable multiple laser beams are used with a single laser crystal in such a manner that multiple pulses can be applied to the substrate.

Description

A laser micromachining system for writing a pattern onto a substrate using laser micromachining and method

The present invention is related to a laser micromachining system for writing a pattern onto a substrate using laser micromachining. It is further related to a laser micromachining method.

Laser micromachining is a technique by which objects or the surface thereof, can be modified or shaped using laser light. For instance, applying light of sufficient energy allows material to be ablated. Due to its inherent moderate spot size, laser light can be used to fabricate objects having very small features.

Figure 1 illustrates a known setup for a laser micromachining device. This device comprises a seed laser 1 capable of generating a plurality of light pulses having an intensity, pulse width and pulse repetition rate. Typically the seed laser emits pulses using a steady repetition rate, i.e. at constant frequency in the order of 30-50MHz, although for some seed lasers this rate can be adjusted during operation. The intensity of the pulse corresponds to the amount of photons per unit time whereas the pulse width corresponds to the duration of the pulse in time. The energy of each photon is well defined due to the narrow bandwidth of the laser light. Consequently, the energy imparted to the substrate is substantially proportional to the product of pulse width and pulse intensity. This energy determines to a large extent the speed at which material is ablated from the surface . A laser crystal 2 is optically coupled to seed laser 1 and is configured to amplify the light pulse. Several materials can be used as laser crystal for instance yttrium aluminium garnet (YAG) material doped with rare-earth ions such as neodymium, ytterbium, or erbium.

Laser crystal 2 can be used to amplify incoming light pulses by means of stimulated emission. In this process an atomic electron (or an excited molecular state), which interacts with an electromagnetic wave of a certain frequency, may drop to a lower energy level, transferring its energy to that field. A photon created in this manner has the same phase, frequency, polarization, and direction of travel as photons of an incident wave.

For this process to result in a net gain of the crystal, a population inversion is required. This can be achieved by supplying energy to the crystal such that the relevant electrons move to higher states. In figure 1, a pumping unit 3 is illustrated for pumping laser crystal 2 with a pumping power. This energy can be in the form of infrared light having a different frequency than the light to be amplified.

The light output of laser crystal 2 is fed to a directing unit 4 for directing the amplified light pulse onto substrate 5. This unit 4 normally comprises a plurality of mirrors and/or lenses.

In a typical mode of operation, a single pulse is applied at a single position on the substrate. The next pulse is then applied at a further position. Directing unit 4 comprises mechanical components with a non-zero mass. Consequently, inertia of directing unit 4 limits the pulse repetition rate of the incoming light pulses which directing unit 4 can handle in such a manner that each pulse is directed to its target position on substrate 5. Typically, the maximum repetition rate of prior art systems is in the order of 100-200kHz.

Because the maximum repetition rate of directing unit 4 and seed laser 1 differ considerably, light pulse selector units 6, 7 are used. Selector unit 6, arranged in between laser crystal 2 and seed laser 1, selects one or more pulses among the plurality of pulses making up the laser beam such that the repetition rate is effectively lowered. For instance, by selecting 1 out of every 200 pulses of a 40MHz laser beam, a repetition rate of 200kHz is obtained. Pulses that are not selected are directed away from laser crystal 2 .

Selector unit 7 is normally used for selecting which amplified pulse should be applied to the substrate. Pulses that are not used for writing the pattern are directed away from substrate 5.

To maintain stable operation, laser crystal 2 is fed with a constant or substantially constant amount of light pulses. Consequently, on average, a constant amount of power is outputted by laser crystal 2. A suitable quantity of pumping power is supplied by pumping unit 3 to maintain this balance. In case less light pulses are provided to laser crystal 2 per unit time, albeit using the same pumping power, the intensity per pulse will increase. Similarly, providing more pulses will result in a decrease in intensity. In most applications, the energy per pulse should be kept as constant as possible. It is therefore common practice, if a pulse need not be applied to the substrate, to discard the pulse after amplification instead of discarding the pulse before amplification. A control unit 8 is used for controlling the directing unit, the pumping unit, and the selector units 6, 7. If at the write position a relatively large depression in the substrate is required, light can be applied using a single high intensity pulse, using a series of pulses having a lower intensity, or using a pulse with a lower intensity albeit for a longer period of time, i.e. having a larger pulse width. If required, control unit 8 may therefore also control seed laser 1 to adjust pulse width, pulse intensity and/or repetition rate. A continuing trend in the art is to provide systems with increasing processing speeds. However, all known solutions are hampered by the fact that the speed of the directing unit is limited.

It is therefore an object of the present invention to provide a system that allows faster processing speeds.

According to the present invention, this object is achieved with a system in accordance with claim 1.

The system according to the invention comprises a seed laser system for outputting a plurality of laser beams, each laser beam comprising a plurality of subsequent light pulses. It further comprises a laser crystal for amplifying light pulses and a pumping unit for providing pumping power to the laser crystal. A first selector unit is provided that is configured for individually switching the laser beams outputted by the seed laser system between an active and inactive state, wherein, the first selector unit is configured to select, for each laser beam in the active state, a light pulse among the plurality of light pulses, to be passed towards the laser crystal for amplification. The system also comprises a light directing unit for directing incident light pulses towards the substrate. A second selector unit is provided that is configured for selecting, for each laser beam outputted by the laser crystal, an amplified light pulse among the plurality of amplified light pulses, to be directed to the light directing unit for writing the pattern. The system also comprises a control unit for controlling the pumping unit, the light directing unit and the first and second selector units.

In prior art systems, a pattern is written using a single laser spot. By combining several spots in a single write event, using the same directing element, a considerable improvement in speed can be obtained. By allowing the laser beams to be individually switchable between active and inactive states, suitable flexibility can be obtained.

The system may comprise a memory coupled to the control unit, the memory storing a write sequence for writing the pattern, the write sequence comprising a sequence of write frames, each write frame having a plurality of write fields corresponding to the plurality of laser beams, the write fields indicating whether a pulse of the relevant laser beam should be applied at a position on the substrate associated with the write frame. The control unit can be configured to control at least one of the pumping unit, the light directing unit and the first and second selector units in accordance with the write sequence. The memory may be at the same location as the control unit hardware. It may also be on a remote location but being accessible by the control unit or it may be an integral component of the control unit. A write frame comprises information whether pulses of the plurality of laser beams should be applied at a particular position.

The present invention particularly relates to embodiments wherein light pulses are applied at discrete positions. Embodiments in which the directing unit moves during application of the pulse, thereby scanning the substrate, are however not excluded. A write frame is typically also associated with a time during which the pulses are to be applied to the substrate.

During this time, the directing unit preferably directs the pulses to the same position. The directing unit can be configured to move to the next position when the pulse has been applied, i.e. after a period corresponding to the pulse width has expired. A suitable delay can be implemented.

It should be noted that typical pulse widths are in the order of 10 picoseconds, which is considerably smaller than the period of the light pulses, typically being in the order of microseconds. It may therefore also be possible to select consecutive light pulses from the seed laser to construct a small burst of light pulses. This simulates the situation in which a single pulse with a higher intensity is applied.

This burst of pulses is applied to the same position.

The memory may further store state information indicating the active or inactive state for each of the plurality of laser beams. This state information can be stored for each write frame. It allows the discrimination between a write field indicating that a pulse should be discarded by the second selector unit (active laser beam, pulse need not be applied) or that a pulse should be discarded by the first selector unit (inactive laser beam). The control unit can use this information to control the selector units accordingly.

Each write frame can be associated with a power and/or energy input/output of the amplified pulses incident on or emerging from the laser crystal, respectively. The write sequence can be divided into write groups, wherein write frames in a single write group correspond to substantially the same power and/or energy input/output. The power and/or energy level can be used by the control unit to control the pumping unit to provide a suitable amount of pumping power. In some embodiments, the power and/or energy information is used to maintain a substantially constant amount of energy contained within a single pulse, regardless the amount of active laser beams. It should be noted that various other physical parameters may be used for the same purpose, such as intensity and/or luminance values.

The write sequence can be ordered based on the power and/or energy input/output. The control unit can be configured to control the pumping unit, the directing unit and the selector units, all or individually, in accordance with the order of the write sequence. For instance, the write sequence may start with write frames having a high associated power and/or energy level and end with write frames having a low associated power and/or energy level.

The control unit can be configured to adjust the pumping power in dependence of the power and/or energy input/output corresponding to a write group. During the course of writing the pattern, the control unit may in the above case lower the pumping power to maintain substantially constant pulse energy .

The control unit may be configured to implement a waiting period in between write groups before controlling the second selector unit to allow light pulses to be directed towards the substrate to allow the intensity of the amplified light pulses to stabilize. The energy and/or power level between write groups may differ considerably. This particularly applies to write groups in which the same energy per pulse is used, but which differ in the amount of active laser beams. In such case it is required to adjust the pumping power. Such adjusting requires time, far exceeding the pulse widths involved. By using a suitable waiting period the situation can be avoided wherein a light pulse having an undefined or unsuitable energy level is applied to the substrate.

In prior art systems, pulses are normally applied in a continuous left to right or vice versa manner. According to the invention however, because multiple laser beams are used, it may be advantageous to first process write frames having the same amount of active laser beams and/or the same power/energy levels even if the positions on the substrate corresponding to those write frames are far apart. In this manner, unnecessary waiting periods due to the changing of energy/power levels can be avoided.

Write frames belonging to different groups may have different power and/or energy levels and typically relate to a different amount of active laser beams. However, it is possible for two write frames to have the same energy/power level although with a different amount of active laser beams. For instance, a write frame may relate to three active beams each carrying 50W, or to two active beams each carrying 75W. These write frames may be attributed to different write groups. Furthermore, suitable waiting periods as defined before may be implemented when switching between different amounts of active laser beams even though the energy/power levels remain substantially the same.

The first and/or second selector unit can also be configured for attenuating incident light pulses. The level of attenuation can be controlled by the control unit. This allows an even further control over the energy that is imparted to the substrate. It may even be possible to implement detectors near the selector units which measure the pulses that are to be discarded. Information on the energy/power levels of these pulses can be used to control the level of attenuation of the pulses that are to be applied to the substrate or to control the pumping power.

The directing unit may comprise a refractive or reflective directing element for refracting or reflecting the light pulse(s) from the laser crystal onto the substrate. The directing element may be rotatable in at least one direction for changing a position on the substrate onto which the light pulses from the laser crystal should be applied. Such directing element can comprise a mirror which is individually rotatable about two axes.

The laser beams preferably falls onto and emerges from the laser crystal in a linear spaced apart manner. For instance, the output and/or input of the laser crystal may comprise a plurality of laser spots evenly distributed on a row. Light pulses may, apart from or alternatively to being separated in space, also be separated in polarization. It may also be possible to alternatively or additionally separate the pulses in time. This is made possible because the pulse widths are very small compared to the period of the light pulse train. In this way, pulse can still occupy the same space and/or share the same optics, albeit not simultaneously. Of course, and not only for such embodiment, it is advantageous to ensure proper synchronization between the sources of the laser beams in the seed laser system.

In other embodiments, the laser beams are combined to occupy substantially the same space prior to entering the laser crystal. While in the crystal, the laser beams can then be amplified as a whole, while after emerging the laser crystal, the individual light pulses are spatially separated again .

According to a second aspect, the present invention provides a method for writing a pattern onto a substrate using laser micromachining in accordance with claim 15.

Next, the invention will be described in more detail using the appended figures, in which:

Figure 1 illustrates a prior art laser micromachining system;

Figure 2 shows an embodiment of a laser micromachining system according to the invention;

Figures 3A-3C shows an example of a structure on a substrate to be fabricated using the system of figure 2;

Figures 4A-4B show two examples of write sequences that can be used for fabricating the structure in figures 3A-3C; and

Figure 5 illustrates a further structure that can be fabricated using the system in figure 2.

Figure 2 shows an embodiment of a laser micromachining system according to the invention. Here, seed laser system 11 comprising three seed lasers 11' emits individual laser beams a, b, c, that are characterized by an intensity, a pulse width, and a pulse repetition rate. Laser beams a, b, c fall onto a laser crystal 12. Laser crystal 12 can be embodied as a crystal slab, covered on either side with heatsinks. The faces of the crystal can be made reflective thereby allowing light pulses to cross the crystal back and forth to gain intensity. Slab crystals are known in the art.

Pumping unit 13 provides pumping power to crystal slab 12 in the form of infrared light. This light can be supplied to crystal slab 12 in a manner similar to light beams a, b, c. More in particular, when the frequencies of laser beams a, b, c and the infrared light are sufficiently far removed, interference will not be an issue allowing laser beams a, b, c and the infrared light to spatially overlap. Other kinds of energy could be used to cause population inversion in crystal slab 12.

Amplified light pulses contained in laser beams A, B, C emerging from crystal slab 12 fall onto a mirror surface 14' of directing unit 14 which can be rotated along two axes. This allows a pattern to be written on substrate 5. It is also be possible to use two separate mirror surfaces which are each individually controllable, albeit each along one axis only.

Control unit 18 controls motors (not illustrated) by which the rotation of mirror surface 14' can be realized. It may further control seed laser system 11 such that at least one of the intensity, pulse width and pulse repetition rate can be adjusted. In addition, it controls pumping unit 13 to control the amount of pumping power that is supplied to crystal slab 12, and selector units 16, 17 as will described in more detail below.

Figures 3A-3C show an example of a structure in substrate 5 that can be written using the system of figure 2. Starting from the blank surface of substrate 5 illustrated in figure 3A, a recess 20 is first created as indicated in figure 3B. A further recess 21 is subsequently created. This latter recess, which is roughly twice as deep as recess 20, can be realized in different ways as will be described next.

Figure 4A illustrates a write sequence 30 that can be used to fabricate the structure of figure 3C. Write sequence 30 comprises two write groups 31, 32. Each write group comprises write frames 33. Each write frame is associated with a position 34 on the substrate. This position information may be part of the write frame. Each write frame 33 comprises three different write fields 35 that are associated with laser beams a, b, c, respectively. In figure 4A, a "1" and a "0" indicate that a pulse should or should not be applied, respectively, at the associated position 34. Furthermore, an "A" and "i", indicate whether a laser beam is active or inactive, respectively.

In figure 4A only the x-coordinate is indicated and is related to the orientation as depicted by arrow 22 in figure 3C.

Figure 4A further illustrates the pumping power that is to be supplied when processing the various write frames. Moreover, the write frames are processed in time corresponding to the order in which they are presented in figure 4A, starting from the left.

First, laser beams a, b, c are all active and are allowed to fall onto substrate 5. In other words, first selector unit 16 selects pulses among the plurality of pulses outputted by the various seed lasers 11' of seed laser system 11. For instance, selector unit 16' selects 1 out of every 200 pulses to obtain a pulse repetition rate of 200kHz starting with a pulse repetition rate of 40Mhz of the laser light outputted by seed laser 11'. Furthermore, the selector units 17' of selector unit 17 all allow the light pulses of laser beams A, B, C to fall onto substrate 5.

Due to the energy imparted to substrate 5, material will be ablated. By repeating this process at three adjacent positions xl, x2, x3, a groove is obtained.

In figure 4A, the first three write frames belong to a single write group 31. The next group 32 comprises different write frames 33. In this write group B, the pumping power has been lowered, for illustrational purposes to 33%, to allow the single active laser beam to have substantially the same energy per pulse as the pulses of write group 31.

Because recess 21 is roughly three times as deep as recess 20, more power and/or time is needed. In write sequence 30 of figure 4A, this is achieved by using the same pulse energy three times at the same position.

Figure 4B shows a different approach of obtaining recess 21. Instead of lowering the pumping power, substantially the same pumping power is maintained when patterning recess 21. Therefore, the energy contained in a single pulse is higher than in write group 31. Write frames 33 in write group 40 also indicate that only laser beam B is active, but only a single write frame 33 is needed to achieve the required depth due to the increased energy levels of the pulses.

Figure 4C shows an even different approach of obtaining recess 21. Compared to figure 4A, the distinction between write groups 31 and 32 has gone as every write frame indicates that all laser beams are active. However, during the last nine write frames, selector unit 17 is operated to discard the amplified pulses belonging to laser beams A and C. Hence, in this embodiment, the laser crystal 12 is not affected in any manner by the processing of substrate 5 as the incident and emerging light pulses are identical during the entire process of patterning substrate 5.

Figure 5 illustrates a different structure that can be written with the system of figure 2. It comprises two rectangular recesses 50 that are connected by a connecting recess 51. It is assumed that recesses 50 are twice as deep as recess 51 and that the width of recess 51 corresponds to the size of a laser spot of a single laser beam. In traditional systems, such structure would be written using a single laser. Recess 50 would be written for instance by applying two consecutive pulses at the same location to allow for the required depth to be achieved. Recess 51 could be written using a single pulse per location.

According to the invention, recess 51 can be written using four active laser beams although only one is used for writing recess 51. Recess 50 can be written using two active laser beams albeit with twice the energy per pulse. The writing of recesses 50 and 51 will substantially require the same amount of pumping power. Hence, relatively long stabilization times associated with changing pumping power can be avoided. This flexibility allows structures to be written more efficiently and faster than what is achievable using prior art systems.

It should be apparent to the skilled person that other embodiments are possible of this invention, the scope of which is defined by the appended claims.

Claims

A system for writing a pattern on a substrate using micromachining, the system comprising: a power supply laser system for performing a plurality of laser beams, each laser beam comprising a plurality of successive light pulses; a laser crystal for amplifying light pulses; a pump unit for providing pump power to the laser crystal; a first selection unit adapted to individually switch the laser beams executed by the feed laser system between an active and inactive state, the first selection unit being adapted to select a light pulse from the plurality of light pulses for each laser beam in the active state to being passed to the laser crystal for amplification; a light directing unit for directing incident light pulses to the substrate; a second selection unit for selecting, for each laser beam executed by the laser crystal, an amplified light pulse from the plurality of amplified light pulses, to be directed to the light alignment unit for writing said pattern; a control unit for controlling the pump unit, the light directing unit and the first and second selection units.

The system of claim 1, further comprising a memory coupled to said control unit, said memory storing a write sequence for writing said pattern, the write sequence comprising a sequence of write frames, each write frame having a plurality of write fields includes those corresponding to the plurality of laser beams, the write fields indicating whether a pulse of the relevant laser beam should be applied to a position on the substrate associated with the writing frame; wherein the control unit is arranged to control at least one of the pump unit, the light directing unit and the first and second selection units in accordance with the writing sequence.

The system of claim 2, wherein said memory further stores state information indicating the active or inactive state for each of the plurality of laser beams.

The system of claim 3, wherein the state information is stored for each write frame.

The system of any one of claims 2-4, wherein the state information is used by the control unit for controlling the first selection unit and wherein, if a write field indicates that an amplified pulse should be applied to the substrate with the laser beam in the active state, the control unit controls the second selection unit accordingly.

A system according to any of claims 2-5, wherein each write frame is associated with a power and / or energy input / output of the amplified pulses that fall or come out of the crystal, respectively, and wherein the write sequence is divided into write groups, wherein the write frames in a single write group correspond to substantially the same power and / or energy input / output.

The system of claim 6, wherein the write sequence is ordered based on the power and / or energy input / output.

A system according to claim 7, wherein the control unit is adapted to adjust the pump power in dependence on the power and / or energy input / output corresponding to a write group.

The system of claim 8, wherein the control unit is adapted to implement a wait time between write groups prior to controlling the second select unit to allow light pulses to be directed to the substrate to allow the intensity of the amplified light pulses to stabilize .

The system of any one of claims 2 to 9, wherein the write sequence comprises write frames associated with the same energy and / or power output, but with different amounts of active laser beams.

11. System as claimed in any of the foregoing claims, wherein the first and / or second selection unit is also adapted to weaken incident light pulses.

A system according to any of the preceding claims, wherein the aiming unit comprises a refractive or reflective aiming element for breaking or reflecting the light pulse (s) of the laser crystal on the substrate, said aiming element being rotatable in at least one direction for changing a position on the substrate to which the light pulses of the laser crystal are to be applied.

The system of claim 12, wherein said aiming element comprises a mirror which is individually rotatable about two axes.

A system according to any one of the preceding claims, wherein the laser beams fall on and come out of the laser crystal in a linear and spaced manner.

A method for writing a pattern on a substrate using micromachining, said method comprising the steps of: performing a plurality of laser beams, each laser beam comprising a plurality of consecutive light pulses; providing a laser crystal for amplifying light pulses; providing pump power to the laser crystal; individually switching the laser beams output from the feed laser system between an active and inactive state, wherein for each laser beam in the active state a light pulse from the plurality of light pulses is selected using a first selection unit to be transmitted to the laser crystal for reinforcement; using a second selection unit to select an amplified light pulse from the plurality of amplified light pulses for each laser beam executed by the laser crystal; directing the selected amplified light pulses to the substrate for writing said pattern.

The method of claim 15, further comprising storing a write sequence for writing said pattern, the write sequence comprising a sequence of write frames, wherein each write frame comprises a plurality of write fields corresponding to the plurality of laser beams, the write fields indicating whether a pulse of the relevant laser beam should be applied at a position on the substrate associated with the writing frame; wherein the method further comprises controlling at least one of the pumping unit, the light directing unit and the first and second selection units in accordance with the writing sequence.

The method of claim 16, further comprising storing state information indicating the active or inactive state for each of the plurality of laser beams.

The method of claim 17, wherein the state information is stored for each write frame.

A method according to any of claims 16-18, comprising controlling the first select unit using the state information and, if a write field indicates that an amplified pulse should be applied to the substrate with the laser beam in the active state, it accordingly controlling the second selection unit.

A method according to any of claims 16-19, wherein each writing frame is associated with a power and / or energy input / output of the amplified pulses falling or coming out of the crystal, respectively, and wherein the write sequence is divided into write groups, wherein the write frames in a single write group correspond to substantially the same power and / or energy input / output.

The method of claim 20, wherein the write sequence is ordered based on the power and / or energy input / output.

The method of claim 21, comprising adjusting the pump power in dependence on the power and / or energy input / output corresponding to a write group.

The method of claim 22, comprising implementing a wait time between write groups prior to controlling the second select unit to allow light pulses to be directed to the substrate to allow the intensity of the amplified light pulses to stabilize.

The method of any one of claims 16 to 23, wherein the write sequence comprises write frames associated with the same energy and / or power output, but with different amounts of active laser beams.

The method of any of claims 15-24, further comprising attenuating incident light pulses using the first and / or second select unit.

A method according to any of claims 15-25, wherein the laser beams fall on and come out of the laser crystal in a linear and spaced manner.